As one of control methods of internal combustion engines, there is known a method which determines a manipulated variable of each actuator with torque and an air-fuel ratio as control variables. For example, Japanese Patent Laid-Open No. 2010-7489 discloses a method which determines a required torque and a required air-fuel ratio for an internal combustion engine, and determines respective manipulated variables of a throttle, an ignition device and a fuel injection device so as to realize them. With regard to a throttle, a throttle opening which is a manipulated variable thereof is determined in accordance with a target air quantity for realizing required torque. For example, with use of an inverse model of an air model, the throttle opening which is necessary for realizing the target air quantity can be obtained by calculation.
Incidentally, in addition to the quantity of the air which is taken into a cylinder, an air-fuel ratio is closely related to the torque which is generated by an internal combustion engine. When air quantity is the same, torque decreases if the air-fuel ratio of the mixture gas which is provided for combustion is leaner than stoichiometry, and torque increases if the air-fuel ratio is rich. Accordingly, in the process of converting the required torque into the target air quantity, the air-fuel ratio of the mixture gas in the cylinder, that is, the required air-fuel ratio is desirably referred to. By setting the target air quantity in accordance with the required air-fuel ratio, precision of realization of the required torque can be enhanced.
However, the required air-fuel ratio is not always constant, and is sometimes positively changed from the viewpoint of exhaust gas performance. For example, at the time of return from fuel cut, the required air-fuel ratio is made significantly richer than stoichiometry for a predetermined time period in order to recover the NOx reduction ability of a catalyst quickly. Further, in order to enhance the purifying performance of the catalyst, the required air-fuel ratio is periodically changed with stoichiometry as the center, and the required air-fuel ratio is changed by air-fuel ratio feedback control. In these cases, the target air quantity also changes in correspondence with change of the required air-fuel ratio, and the throttle opening is also controlled in correspondence with it. The movement of the throttle at this time becomes such movement as to cancel out the variation of torque accompanying the change of the air-fuel ratio by increase/decrease of the air quantity. More specifically, when the air-fuel ratio changes to a rich side, the throttle moves to a closing side to cancel out the increase in torque due to the change by decrease in the air quantity. Conversely, when the air-fuel ratio changes to a lean side, the throttle moves to an opening side to cancel out the decrease in torque due to the change by increase in the air quantity.
However, there is a delay in the response of the air quantity to the movement of the throttle, and the actual air quantity changes late with respect to the change of the target air quantity. Accordingly, when a sudden change occurs to the required air-fuel ratio, change of the air quantity does not catch up with the change of the required air-fuel ratio. As a result, the following problem occurs.
FIG. 3 is a diagram showing each change with time of torque, an engine speed, an air-fuel ratio, a fuel injection quantity, a cylinder intake air quantity and a throttle opening when the required air-fuel ratio abruptly changes, in chart. In the chart of each stage, the dotted line represents a change with time of a required value or a target value of each item, and the solid line represents an actual behavior of each item. As shown in the diagram, when the required air-fuel ratio abruptly changes to a lean side stepwise, the target air quantity also abruptly increases stepwise in response thereto. However, since the throttle opening cannot be increased stepwise, and response of the air quantity is delayed with respect to the movement of the throttle, the actual air quantity increases later than the target air quantity.
Since the fuel injection quantity is determined by the actual air quantity and the required air-fuel ratio, the fuel injection quantity temporarily decreases significantly due to a delay in increase of the air quantity. As a result, the torque generated by the internal combustion engine temporarily reduces significantly with respect to the required torque, and the engine speed also temporarily reduces significantly. With this, a variation also occurs to the actual air-fuel ratio. According to the art described in Japanese Patent Laid-Open No. 2010-7489, when the actual torque may become larger than the required torque, retardation of the ignition timing is performed so as to compensate for the deviation. However, since retardation of the ignition timing causes worsening of fuel economy, the ignition timing is desired to be kept at the optimal ignition timing as far as possible, from the viewpoint of fuel economy performance. However, when such a desire is to be satisfied, temporary reduction in the torque and the engine speed occurs when the required torque abruptly changes to a lean side as shown in FIG. 3.
In conclusion, the aforementioned conventional control method has a room for further improvement in satisfying the requirement concerning the exhaust gas performance of the internal combustion engine, the requirement concerning the fuel economy performance, and the requirement concerning the operation performance with an excellent balance.